Process for a dual extract flush

ABSTRACT

The present invention relates to dual extract flush for feeds for xylene extraction processes. More specifically, the present invention relates to dual extract flush for feeds for simulated moving bed extraction processes. It decouples line flush in and line flush out, providing a means for optimizing each flush independently. This scheme will allow for minimizing each bedline and flushing each according to its own bedline volume, which will minimize any additional non-ideal compositions added to the chambers or downstream fractionation.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application Ser.No. 62/747,900 filed Oct. 19, 2018, the entirety of which isincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to dual extract flush of feeds for xyleneextraction processes. More specifically, the present invention relatesto dual extract flush of feeds for xylene extraction processes such aspara-xylene extraction or meta-xylene extraction by use of a simulatedmoving bed.

BACKGROUND OF THE INVENTION

In the para-xylene and meta-xylene simulated bed separation process, andother simulated moving bed separation processes, bedlines are used tocarry both extracted and unextracted streams. This means that thesebedlines need to be flushed clean of unextracted components before theextracted streams can be withdrawn. If this is not done, the purity ofthe product will be impacted. In the current practice, the bedline whichhas been used to carry feed is flushed twice. The first time, after thefeed has progressed two bedlines down, the residual feed material isflushed into the chambers and displaced with extract material from theopposite end of the adsorption cycle. The second flush pushes theextract material that is now in the bedline, along with any residualcontaminants that weren't flushed initially, to the bed directly belowwhere extract is being withdrawn.

There are several problems with the current practice. First, it couplesthe requirement of line flush out with the initial line flush in, thoughthese two may not have the same optimal settings (i.e. line flush outmay be optimal at approximately 100%, whereas line flush in may beoptimal at >100%). Second, it leaves a bedline full of desorbent thatwill be the first bedline volume worth of material sent to the extractcolumn, which is inefficient for separation, because heat is used toseparate this material, though it has already been separated. Third, anyfeed that remained in the feed bedline is added to the chambers two fullbeds after the feed goes in, 2-3 minutes after the feed was introduced,at a point where some separation has already occurred, which introducesan inefficiency in feed separation. Finally, any residual contaminantsfrom the feed that remain in the bedline after the first flush are sentto the bed directly below the extract, so there is only one bed ofseparation to remove these contaminants from the extract.

SUMMARY OF THE INVENTION

The present disclosure encompasses several benefits. It decouples lineflush in and line flush out, providing a means for optimizing each flushindependently. This scheme will allow for minimizing each bedline andflushing each according to its own bedline volume, which will minimizeany additional non-ideal compositions added to the chambers ordownstream fractionation. It is also unique as it will use extractmaterial as both the first flush and the second flush. This is importantbecause the remaining bedline material that will be bedline right beforeit is sent to the extract column will have extract in it, so this willbe processed normally by the extract column, without the requirement ofadditional duty for processing extra desorbent.

Another benefit is that it has the provision for a second flush whichwill ensure the bedline is clean and send this material to a locationbetween the first flush and the extract where the composition mostclosely matches what is in the bedline. With the current design withsecond flush, this is required because of contamination that occurs atthe rotary valve due to stepping of the rotary valve. A recent inventionprevents this contamination, so second flush for this purpose won't benecessary. The second flush of this design will only be to add what isnecessary to keep residual un-flushed contaminants (that remain afterthe first flush) out of the bedline before it is used to carry extract.

This invention will also incorporate relocation of the over-flushedcomponent of the first flush so that it does not negatively impactadsorptive capacity of adsorbent at the feed point in the column. Thecurrent practice is to over-flush the first flush by approximately 20%beyond the bedline volume. This over-flushing introduces desorbent andalready-extracted para-xylene into the feed point, which results in acapacity debit. If this 20% over-flush is required for sake of purity,it should be sent to the chambers closer to the extract point, as thematerial in the beds is closer to that which is in the bedline beingflushed. In this way, the impact on capacity will be reduced, or for allpractical purposes be eliminated. This will be the purpose of the secondextract flush.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by volume, unless otherwise indicated.Other objects, advantages and applications of the present invention willbecome apparent to those skilled in the art from the following detaileddescription and drawing. Additional objects, advantages and novelfeatures of the examples will be set forth in part in the descriptionwhich follows, and in part will become apparent to those skilled in theart upon examination of the following description and the accompanyingdrawing or may be learned by production or operation of the examples.The objects and advantages of the concepts may be realized and attainedby means of the methodologies, instrumentalities and combinationsparticularly pointed out in the appended claims.

BRIEF DESCRIPT OF THE DRAWING

The FIGURE illustrates a dual extract flush for feeds for simulatedmoving bed extraction processes.

DEFINITIONS

As used herein, the term “stream” can include various hydrocarbonmolecules and other substances.

As used herein, the term “stream”, “feed”, “product”, “part” or“portion” can include various hydrocarbon molecules, such asstraight-chain and branched alkanes, naphthenes, alkenes, alkadienes,and alkynes, and optionally other substances, such as gases, e.g.,hydrogen, or impurities, such as heavy metals, and sulfur and nitrogencompounds. Each of the above may also include aromatic and non-aromatichydrocarbons.

As used herein, the term “overhead stream” can mean a stream withdrawnat or near a top of a vessel, such as a column.

As used herein, the term “bottoms stream” can mean a stream withdrawn ator near a bottom of a vessel, such as a column.

Hydrocarbon molecules may be abbreviated C1, C2, C3, Cn where “n”represents the number of carbon atoms in the one or more hydrocarbonmolecules or the abbreviation may be used as an adjective for, e.g.,non-aromatics or compounds. Similarly, aromatic compounds may beabbreviated A6, A7, A8, An where “n” represents the number of carbonatoms in the one or more aromatic molecules.

Furthermore, a superscript “+” or “−” may be used with an abbreviatedone or more hydrocarbons notation, e.g., C3+ or C3−, which is inclusiveof the abbreviated one or more hydrocarbons. As an example, theabbreviation “C3+” means one or more hydrocarbon molecules of three ormore carbon atoms.

As used herein, the term “unit” can refer to an area including one ormore equipment items and/or one or more sub-zones. Equipment items caninclude, but are not limited to, one or more reactors or reactorvessels, separation vessels, adsorbent chamber or chambers, distillationtowers, heaters, exchangers, pipes, pumps, compressors, and controllers.Additionally, an equipment item, such as a reactor, dryer, adsorbentchamber or vessel, can further include one or more zones or sub-zones.

The term “column” means a distillation column or columns for separatingone or more components of different volatilities. Unless otherwiseindicated, each column includes a condenser on an overhead of the columnto condense and reflux a portion of an overhead stream back to the topof the column and a reboiler at a bottom of the column to vaporize andsend a portion of a bottoms stream back to the bottom of the column.Feeds to the columns may be preheated. The top or overhead pressure isthe pressure of the overhead vapor at the vapor outlet of the column.The bottom temperature is the liquid bottom outlet temperature. Overheadlines and bottoms lines refer to the net lines from the columndownstream of any reflux or reboil to the column unless otherwise shown.Stripping columns omit a reboiler at a bottom of the column and insteadprovide heating requirements and separation impetus from a fluidizedinert media such as steam.

As depicted, process flow lines in the FIGURES can be referred tointerchangeably as, e.g., lines, pipes, feeds, gases, products,discharges, parts, portions, or streams.

The term “passing” means that the material passes from a conduit orvessel to an object.

The terms “application” and “program” refer to one or more computerprograms, software components, sets of instructions, procedures,functions, objects, classes, instances, related data, or a portionthereof adapted for implementation in a suitable computer code(including source code, object code, or executable code). The term“communicate,” as well as derivatives thereof, encompasses both directand indirect communication.

The “control system” is defined as hardware and software computingcomponents that determines the set points of control elements in theadsorption section.

The term “Zone 1” is defined as the zone in the adsorbent chambers belowthe feed and above the raffinate points, and may also be referred to asthe “adsorption zone”.

The term “Zone 2” is defined as the zone in the adsorbent chambers belowthe extract and above the feed points, and may also be referred to asthe “purification zone”.

The term “Zone 3” is defined as the zone in the adsorbent chambers belowthe desorbent and above the extract points, and may also be referred toas the “desorption zone”.

DETAILED DESCRIPTION OF THE INVENTION

The following detailed description is merely exemplary in nature and isnot intended to limit the application and uses of the embodimentdescribed. Furthermore, there is no intention to be bound by any theorypresented in the preceding background or the following detaileddescription.

The description of the apparatus of this invention is presented withreference to the attached FIGURE. The FIGURE is a simplified diagram ofthe preferred embodiment of this invention and is not intended as anundue limitation on the generally broad scope of the descriptionprovided herein and the appended claims. Certain hardware such asvalves, pumps, compressors, heat exchangers, instrumentation andcontrols, have been omitted as not essential to a clear understanding ofthe invention. The use and application of this hardware is well withinthe skill of the art.

The various embodiments described herein relate to a process for dualextract flush 10. As shown in the FIGURE, a process 10 comprises of aunit 12 comprising an adsorption zone 18, a purification zone 16 and adesorption zone 14. In simulated moving bed technologies such as, butnot limited to, those for para-xylene separation, each zone and thelocation of each stream that is added or withdrawn from a chamber movesin succession down through the unit 12, and once it reaches the bottomof unit 12, is sent back to the top of unit 12. The relative positionsof each zone and stream remains the same relative to all other zones andstreams. In the example shown in the FIGURE, the adsorption zone 18 islocated in unit 12 between the feed 20 and raffinate 24, thepurification zone 16 is located in unit 12 between the extract 26 andfeed 20, and the desorption zone 14 is located in unit 12 between thedesorbent 22 and extract 26. The adsorption zone includes a feed 20 anda raffinate 24. The raffinate exits the unit 12 at the bottom of theadsorption zone 18. The feed 20 enters the unit 12 at the top of theadsorption zone 18. The purification zone 16 includes an extract 26 thatexits the unit 12 at the top of the purification zone 16, and a firstflush 40 and a second flush 42. The first flush 40 is located below thesecond flush 42. Here, the extract 26 exits the unit 12 and is sent toan extract column by line 38. Line 38 is joined by a line flush out 28and a portion is taken as line 36 to pump 44.

The extract stream 26 includes a portion 37 that is sent to the extractcolumn, and the remaining stream 36 is sent to a pump 44. Stream 38 thatis sent to the extract column also includes the line flush 28 that exitsthe unit 12 in the desorption zone 14 below the desorbent 22.

Once the extract stream 36 passes through the pump 44, it is dividedinto two flush lines. The first flush 40 pumped from line 36 and sent toa bedline in the unit 12 immediately after it was used to bring feedinto the chamber. The bedline still contains feed, and these remainingfeed components will be flushed into the chamber, to send them into thechamber as closely as possible to the location where feed is enteringthe chamber. This flush will be set to flush approximately 80-100% ofthe bedline volume during a single step, with little or no allowance forover-flushing, as over flushing at this location introduces a capacitydebit. The flushing flow rate will be controlled by a control system.

Additional flushing is required for sake of product purity. Thisadditional flush should be sent into the purification zone closer to theextract point, as the composition in the beds near that point are moreenriched in both desorbent and the desired component. Current practiceis to send this material 1 bed below the extract. However, in thisinvention, the over-flushed amount will have a higher concentration offeed contaminants that may cause contamination of the extract product.The rate of this flush should be low, just enough to clear residual feedcomponents. It is contemplated that this may be approximately 20-50% ofa bedline volume. This estimate is taken from modeling work thatsuggests 120-130% of a bedline volume needs to be flushed with flushingmaterial to clear the entire bedline of feed contaminants. Therefore,the remaining required flush, after the initial 80-100% of bedline flushis done with the first flush 40, will accomplished by the second flush42 later in the purification zone 16.

The first flush 40 enters the unit 12 in the purification zone 16 belowthe second flush line 42 which also enters the unit 12 in thepurification zone 16. In one contemplated example, the first flush 40may comprise 60-80% of the total flushing amount required and the secondflush 42 may comprise 40-20% of the total flushing amount required. Inanother contemplated example, the first flush 40 may comprise 70% of thetotal flushing amount required and the second flush 42 may comprise 30%of the total flushing amount required. In another contemplated example,the first flush 40 may comprise 60% of the total flushing amountrequired and the second flush 42 may comprise 40% of the total flushingamount required. In yet another contemplated example, the first flush 40may comprise 50% of the total flushing amount required and the secondflush 42 may comprise 50% of the total flushing amount required.However, it is also contemplated that other combinations of line flushesmay be used.

It is contemplated that the first flush 40 will be located in thepurification zone 16 two bedlines above the feed 20. It is contemplatedthat the second flush 42 will be located in the purification zone 16about 2 beds to about 4 beds above the feed 20.

Implementing this invention would represent a large economic benefit. Itwill reduce the amount of desorbent component in extract stream 38 thatneeds to be separated in the extract column. Also, it will increase theadsorptive capacity of the adsorbent in unit 12.

While the invention has been described with what are presentlyconsidered the preferred embodiments, it is to be understood that theinvention is not limited to the disclosed embodiments, but it isintended to cover various modifications and equivalent arrangementsincluded within the scope of the appended claims.

Specific Embodiments

While the following is described in conjunction with specificembodiments, it will be understood that this description is intended toillustrate and not limit the scope of the preceding description and theappended claims.

A first embodiment of the invention is a process for a dual extractflush, comprising passing a feed and desorbent into a unit wherein theunit comprises a desorption zone, a purification zone, and an adsorptionzone wherein the desorption zone is located between the desorbent andextract, the purification zone is located between the extract and feed,and the adsorption zone is located between the feed and raffinate;removing an extract from the unit and splitting a portion of the extractstream into a first flush and a second flush; and passing the firstflush and the second flush back to the unit. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the first embodiment in this paragraph, wherein the unit is apara-xylene extraction unit. An embodiment of the invention is one, anyor all of prior embodiments in this paragraph up through the firstembodiment in this paragraph, wherein the unit is a meta-xyleneextraction unit. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the first embodiment inthis paragraph, wherein the feed enters the unit at a meeting point ofthe purification zone and the adsorption zone. An embodiment of theinvention is one, any or all of prior embodiments in this paragraph upthrough the first embodiment in this paragraph, wherein the extract isremoved from the unit a meeting point of the desorption zone and thepurification zone. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the first embodiment inthis paragraph, wherein the first flush enters the unit below the secondflush. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph, wherein the first flush and the second flush enter the unitabove the feed but below the extract. An embodiment of the invention isone, any or all of prior embodiments in this paragraph up through thefirst embodiment in this paragraph, wherein the first flush may comprise70% of the flush and the second flush may comprise 30% of the flush. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph,wherein the first flush may comprise 60% of the flush and the secondflush may comprise 40% of the flush. An embodiment of the invention isone, any or all of prior embodiments in this paragraph up through thefirst embodiment in this paragraph, wherein the first flush may comprise80% of the flush and the second flush may comprise 20% of the flush. Anembodiment of the invention is one, any or all of prior embodiments inthis paragraph up through the first embodiment in this paragraph,wherein the first flush and the second flush are controlled by a controlsystem. An embodiment of the invention is one, any or all of priorembodiments in this paragraph up through the first embodiment in thisparagraph, wherein the first flush and the second flush may becontrolled independently. An embodiment of the invention is one, any orall of prior embodiments in this paragraph up through the firstembodiment in this paragraph, further comprising combining the extractproduct stream from the unit, with a line flush out and sending thiscombined stream to an extract column. An embodiment of the invention isone, any or all of prior embodiments in this paragraph up through thefirst embodiment in this paragraph, wherein the unit comprises aplurality of beds, and the second flush enters the unit two to four bedsabove the feed. An embodiment of the invention is one, any or all ofprior embodiments in this paragraph up through the first embodiment inthis paragraph, wherein the unit comprises a plurality of beds, and thefirst flush enters the unit two beds above the feed. An embodiment ofthe invention is one, any or all of prior embodiments in this paragraphup through the first embodiment in this paragraph, further comprising athird flush that may be split from the extract product stream from theunit and sent back to the unit in the purification zone. An embodimentof the invention is one, any or all of prior embodiments in thisparagraph up through the first embodiment in this paragraph, wherein thethird flush may enter the unit in between the second flush and theextract.

A second embodiment of the invention is a process for a dual extractflush, comprising passing a feed and a desorbent to a unit comprising aplurality of beds wherein the unit comprises a desorption zone, apurification zone, and an adsorption zone wherein the desorption zone islocated between the desorbent and extract, the purification zone islocated between the extract and feed, and the adsorption zone is locatedbetween the feed and raffinate; removing an extract from the unit;splitting the extract stream into a first flush and a second flush; andpassing the first flush and the second flush back to the unit whereinthe first flush may comprise 70% of the flush and the second flush maycomprise 30% of the flush. An embodiment of the invention is one, any orall of prior embodiments in this paragraph up through the secondembodiment in this paragraph, wherein the unit comprises a plurality ofbeds, and the second flush enters the unit below the extract and two tofour beds above the feed. An embodiment of the invention is one, any orall of prior embodiments in this paragraph up through the secondembodiment in this paragraph, wherein the unit comprises a plurality ofbeds, and the first flush enters the unit below the extract two bedsabove the feed.

Without further elaboration, it is believed that using the precedingdescription that one skilled in the art can utilize the presentinvention to its fullest extent and easily ascertain the essentialcharacteristics of this invention, without departing from the spirit andscope thereof, to make various changes and modifications of theinvention and to adapt it to various usages and conditions. Thepreceding preferred specific embodiments are, therefore, to be construedas merely illustrative, and not limiting the remainder of the disclosurein any way whatsoever, and that it is intended to cover variousmodifications and equivalent arrangements included within the scope ofthe appended claims.

In the foregoing, all temperatures are set forth in degrees Celsius and,all parts and percentages are by weight, unless otherwise indicated.

What is claimed is:
 1. A process for a dual extract flush, comprising:passing a feed and desorbent into a unit wherein the unit comprises adesorption zone, a purification zone, and an adsorption zone wherein thedesorption zone is located between the desorbent and extract, thepurification zone is located between the extract and feed, and theadsorption zone is located between the feed and raffinate; removing anextract from the unit and splitting a portion of the extract stream intoa first flush and a second flush; and passing the first flush and thesecond flush back to the unit.
 2. The process of claim 1, wherein theunit is a para-xylene extraction unit.
 3. The process of claim 1,wherein the unit is a meta-xylene extraction unit.
 4. The process ofclaim 1, wherein the feed enters the unit at a meeting point of thepurification zone and the adsorption zone.
 5. The process of claim 1,wherein the extract is removed from the unit at a meeting point of thedesorption zone and the purification zone.
 6. The process of claim 1,wherein the first flush enters the unit below the second flush.
 7. Theprocess of claim 1, wherein the first flush and the second flush enterthe unit above the feed but below the extract.
 8. The process of claim1, wherein the first flush may comprise 70% of the flush and the secondflush may comprise 30% of the flush.
 9. The process of claim 1, whereinthe first flush may comprise about 60% to about 80% of the flush and thesecond flush may comprise about 40% to about 20% of the flush.
 10. Theprocess of claim 1, wherein the first flush may comprise 80% of theflush and the second flush may comprise 20% of the flush.
 11. Theprocess of claim 1, wherein the first flush and the second flush arecontrolled by a control system.
 12. The process of claim 1, wherein thefirst flush and the second flush may be controlled independently. 13.The process of claim 1, further comprising combining the extract productstream from the unit, with a line flush out and sending this combinedstream to an extract column.
 14. The process of claim 1, wherein theunit comprises a plurality of beds, and the second flush enters the unittwo to four beds above the feed.
 15. The process of claim 1, wherein theunit comprises a plurality of beds, and the first flush enters the unittwo beds above the feed.
 16. The process of claim 11, further comprisinga third flush that may be split from the extract product stream from theunit and sent back to the unit in the purification zone.
 17. The processof claim 16, wherein the third flush may enter the unit in between thesecond flush and the extract.
 18. A process for a dual extract flush,comprising: passing a feed and a desorbent to a unit comprising aplurality of beds wherein the unit comprises a desorption zone, apurification zone, and an adsorption zone wherein the desorption zone islocated between the desorbent and extract, the purification zone islocated between the extract and feed, and the adsorption zone is locatedbetween the feed and raffinate; removing an extract from the unit;splitting the extract stream into a first flush and a second flush; andpassing the first flush and the second flush back to the unit whereinthe first flush may comprise about 60% to about 80% of the flush and thesecond flush may comprise about 40% to about 20% of the flush.
 19. Theprocess of claim 18, wherein the unit comprises a plurality of beds, andthe second flush enters the unit below the extract and two to four bedsabove the feed.
 20. The process of claim 18, wherein the unit comprisesa plurality of beds, and the first flush enters the unit below theextract two beds above the feed.